1. Field of the Invention
The present invention relates to an optical scanning device and a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus.
2. Description of the Background Art
Today, a color image forming apparatus is replacing a monochromatic image forming apparatus in parallel with the spread of personal computers. In this respect, there is an increasing demand for a small-size high-velocity image forming apparatus. A color copier, which is a specific form of an electrophotographic image forming apparatus, is capable of producing a full-color image by using, e.g., yellow, magenta, cyan and black toner. For this purpose, the color copier forms images of different colors either serially or in parallel with respect to time. As for the number of copies of size A4 to be output for a minute, parallel processing produces a greater number of copies than serial processing, as well known in the art.
Small-size, low-cost writing optics is essential with an image forming apparatus of the type executing parallel processing stated above. More specifically, this type of image forming apparatus needs optical writing units equal in number to images of different colors to be dealt with together. Even when a particular optical writing unit is assigned to each image forming section, an LDA (Laser Diode Array) is generally used for size reduction. However, considering performance (pixel density and tonality) required of light-emitting devices, it is impractical to reduce cost more than a certain limit. In this sense, the cost of the individual light-emitting device itself must be reduced. Also, installation cost for implementing the accuracy of joints between nearby light-emitting devices must be reduced. Further, the positional relation between the light-emitting devices and a surface to be scanned must be maintained with accuracy.
Various light-emitting devices are available for optical writing devices used in various fields (LD (Laser Diode), CD (Compact Disk), CD-R (CD ROM), CD-RW (CD Read-Write) and DVD (Digital Versatile Disk)) and are decreasing in cost. Such light-emitting devices reduce the cost of the optical writing unit more than LDAs. However, a light source including a light-emitting device, a deflector for steering a light beam and optical lenses, among others, are the key to cost reduction. It is therefore necessary to reduce the cost of such key parts as well as a space to be occupied thereby.
In light of the above, Japanese Patent Laid-Open Publication Nos. 11-249042 and 2000-214401 each disclose an optical scanning device devised to integrate a deflector and a light source. However, a problem with Laid-Open Publication No. 11-249042 is that only one deflecting surface is available with a deflector. Another problem is that a plurality of light-emitting devices cannot be used in combination because light beams incident to the deflector other than the center of rotation vary in the direction of height. Such an optical scanning device is not feasible for high-velocity scanning. A problem with Laid-Open Publication No. 2000-214401 is that an optical switching device is necessary and increases cost.
There have also been proposed an arrangement in which the deflecting face of a deflector is increased in height, an arrangement in which a deflector has deflecting faces stacked one above the other, and an arrangement in which a plurality of deflecting faces are used. These arrangements, however, not only increase the cost of the deflector, but also obstruct the sharing of a light source and optical devices including fθ lenses.